[RTTY] ae4jy HF path simulator

[Terry Gerdes]

Hi Patrick,

Thanks for posting this.  It reminds me of a decoding issue that I want to
throw out and see if the group has any ideas.  I run four sound cards in the
main station computer.  Two run in the standard MMTTY mode and the other two
can be operated in other MMTTY modes.  The question is what is the best mode
for the other two MMTTY instances?

One issue that I have is working stations who are within 50 to 200 miles of
me.  I can usually get a couple of letters of the call but not enough to
make it a legal QSO.  Since we are close, I suspect we are operating on a
backscatter path.   Any ideas on what would be the best MMTTY mode for this?

Thanks

Terry - AB5K


----- Original Message ----- 
From: <f6irf at free.fr>
To: <rtty at contesting.com>
Sent: Tuesday, November 02, 2004 8:42 AM
Subject: [RTTY] ae4jy HF path simulator


> Don't know if it has already mentionned here, but this is a simple but
extremely
> useful tool to compair decoders, fine-tune MMTTY filters and settings,
etc...
> http://www.qsl.net/ae4jy/pathsim.htm
>
> Works from WAV file - so everyhting can be done on a single PC:
> - First record a RTTY signal, for example using AFSK output of MMTTY as
> generator and pathsim as simple WAV recorder (disable noise generator and
other
> pathes); anything like "THE QUICK BROWN FOX " or a list of callsigns...
(record
> at relatively low level i.e max -6dB or 25% as adding paths later may
overdrive
> the soundcard)
> - Open the WAV-file created with PathSim  and set the output to
soundcard - add
> white noise, multipath, flutter either manually or using predefined
presets,
> and try to decode...
>
> Have fun !
> Patrick
>
>
> Additional comments:
> - Don't miss the "HighLatitudeDisturbed" preset which gives a realistic
polar
> path sounding...
> - It is interesting to compair decoders, but do not forget that you won't
have
> any pre-filtering (what your transceiver is normaly providing -especialy
if you
> have a built-in DSP)- so do not make conclusions too fast: for example
with
> MMTTY the major improvement is obtained by using/narrowing the BPF
filter... In
> the real world the BPF is almost useless if you have a transceiver with
> DSP-filter or if you are using an external DSP).
> - It can also be used to test external devices like DSP filters, or
hardware
> terminals...
> - It can also be used to compair modes (once you have reached the limits
of
> RTTY, you may give a try at PSK, MSFK16 and CW...)
> - Don't forget that the S/N ratio is expressed for a bandwidth of 3kHz -
S/N
> expressed in dBs follows  the usual log base10 progression I.E. S/N -10dB
in
> 3kHz bandwidth is equivalent to 0dB in 300hz BW  -3dB in 600Hz or -6dB in
> 1200Hz or +10dB in 30Hz.
> - To give a reference point Joe, K1JT, Nobel Prize of Physics and author
of
> WSJT,  considers that the limit for CW "human decoding" at 12WPM
is -12dB(*) in
> 2500Hz BW - (roughly -13dB in 3kHz BW - this is a for signal in white
noise or
> AWGN channel) - ref appendix D of WSJT version 3 manual page 50
> http://pulsar.princeton.edu/~joe/K1JT/WSJT300.PDF
>
> (*) the graph page 51 rather shows -14dB, anyway, if you are a CW-op you
can
> try...  Interesting to see what it gives at "equal speed" ie 60WPM (45bd
RTTY
> speed) - the theory says  10log (60/12) or roughly 7dB more signal needed
due
> to speed increase...
> Taking the most favourable (the graph) option, that's make a minimum S/N
level
> of -8dB in 3kHz bandwidth for CW at 60WPM...(-6dB taking the text option).
Now
> you can compair CW with RTTY ! (interesting isn't it?)
>
>
>
>
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